[Daox]

Originally Posted by Talos Woten

QUESTIONS

Would the WRAI described above (behind the radiator) be better than the stock intake for FE at 65 mph cruise control? That question really boils down to:

I do not believe so.

Does anyone know the difference in pressures at the wheel well top vs. behind the radiator? (Julian Edgar, where are you when we need you? I know the MAF measures mass air flow and temperature, so this could be the same as...

I doubt there is much difference at all.

Does the car measure AI pressure? If so, I could just mock up what I'm thinking and measure all the stats directly.

I do not believe so. It just has a MAF, not a MAP sensor.

Can anyone think of issues with doing the WRAI? Right off the bat, it seems to me that we'd be trapping heat in a loop, which should help with cold starts / weather BUT might be dangerous in a heavy load / hot day scenario. Subtleties like ignition timing etc. need to change with the air characteristics, but I'm assuming the fuel injected ECU magically takes care of all that.

It is possible that warm air may be somewhat beneficial for your fuel economy on a Prius, but I am skeptical. On a non-hybrid I would certainly recommend it (and I run one on my Mirage), but the Prius is a bit different. As soon as the Prius' rpm are above 1200, the engine is pretty optimally loaded. RPM is the only differing factor now, and the RPM is changed by vehicle speed and engine load. Lets take a look at a 1nz-fxe brake specific fuel consumption chart to explain this.



This map shows that the Prius' engine is most efficient when operated between 2000-4000 rpm. If you're already in this range on while cruising you're not going to get a ton better. Hotter air should theoretically move the sweet spot on the map to lower rpms if that is what you want while colder air should move it to higher rpms.

Ignition timing is pulled at higher rpms on most engines to protect it. This is all done automatically via the ECU. I'm not sure if that is the case on the Prius' engine because of its atkinson-ish cycle it uses. Its likely the intake charge never really gets hot enough to require this, which would be another perk of the design albeit at the expense of maximum power output.

And for the bonus question: has anyone already built and measured such a device? I came across a ton of DIY mods for CRAIs (Cold Ram Air Intakes) but couldn't find any WRAIs.

I don't believe a ram air intake will do anything for fuel economy. There isn't enough pressure build up at the speeds we travel at, and combine that with the fact it may only be useful during acceleration and you have such a limited case scenario that its just not worth putting time into versus other mods.